Haemodialysis

Quantitation and adequacy

Measurements of small molecule clearance have become an accepted way of assessing the adequacy of haemodialysis, because a correlation with mortality has been shown in large studies. It is a Renal Association standard that adequacy is checked monthly on all chronic haemodialysis patients.

Kt/V for urea* is the most widely accepted of these measurements, although a simple surrogate for this, urea reduction ratio (URR), is more readily measured and is also widely used. There is a calculator for Kt/V in Proton.

* K is a dialyser-specific figure for rate of urea clearance on an individual dialyser, at a given blood flow and dialysate flow. t is duration of dialysis. V is the volume of distribution of urea in a patient.

Urea reduction ratio method

In the RIE we check Urea Reduction Ratio (URR), calculated as:

URR = 100 x [1 – Post(Urea)/Pre(Urea)]

The Edinburgh unit participates in a multicentre audit with comparison of URR results between all units in Scotland. This is the protocol approved by the Scottish Renal Association:

The ‘pre’ sample is taken immediately after cannulation of the fistula with a dry needle and before the dialysis starts. For central venous cannulae, a sample can be drawn from the line after the Heparin lock has been removed. Take care not to contaminate the samples with saline.

The post sample is taken by the stop flow method ie. the dialysate flow is stopped for 5 minutes, leaving the blood pump running, before sampling from the port in the arterial blood line. This is to allow time for the blood in the access to equilibrate with the central circulation, but not enough time for the equilibration from the tissue pool

Only these bloods should be marked PRE (001) and POST (002) and are done by staff in the dialysis Units on a monthly basis.

Bloods taken without this stop flow will overestimate urea clearance on dialysis. Hence, while it may be appropriate to take bloods pre-dialysis on eg. in-patients to save patients venepuncure, and it is often necessary to know post dialysis potassium in out-patients who have not for whatever reason, completed their dialysis prescription, mark these bloods with the time they are taken, not pre/post or the 001/002 codes. This is very important as failure to do so causes problems with download of data to the Registry.

Problems with Kt/V & URR

Small molecule clearance is not the only or even always the most important factor in determining dialysis adequacy.

Fluid balance is important for mortality and is not measured by Kt/V or URR. It is more affected by dialysis frequency and duration.

Calculations that use measurements of blood urea (instead of dialysate urea) are prone to sampling errors, particularly urea rebound.

V (volume of distribution of urea) is low in wasted, malnourished patients – high Kt/V values may be caused by this, rather than by excellent dialysis.

Kt/V versus URR

Calculations based on URR alone underestimate Kt/V because no account is taken of the consequences of ultrafiltration. This removes urea, but is ‘invisible’ by URR monitoring alone.Table: correlation of URR to Kt/V corrected for ultrafiltration (NKF K/DOQI Guideline 2000). Calculated from single-pool, variable volume model with a body weight of 67.3kg, V of 35L, and NPCR of 1.0. Wt/BWt is the ultrafiltration volume/post dialysis weight x 100.

URR Values at Kt/V

DWt/BWt %

0.8

0.9

1.0

1.1

1.2

1.3

1.4

1.5

0

53

57

61

65

68

71

74

76

1

52

57

61

64

68

71

73

76

2

50

55

59

63

66

69

72

75

3

49

54

58

62

65

68

71

74

4

48

53

57

61

64

67

70

73

5

47

51

56

60

63

67

70

72

6

46

50

55

59

62

66

69

71

7

44

49

53

58

61

65

68

71

8

43

48

52

57

60

64

67

70

9

42

47

51

56

59

63

66

69

10

41

46

50

54

58

62

65

68

Prescribed Dialysis:

Dialysis should be prescribed on an individual basis. The variables influencing the amount of dialysis required are:

Weight: Higher weight = more dialysis required. Note that in serial audits, we have previously underprescribed dialysis for large patients

Sex: Men require more than women of the same weight due to greater proportional body water

The variables in the prescription that will improve dialysis adequacy are:

Time: this is the most important variable to achieve adequacy and the time likely to be required needs to be explained to patients BEFORE they choose haemodialysis

Dialysate flow rate (Qd): either 500 or 800ml/min possible (NB the difeerence this makes is not quantified and likely to be small but if Qb is 400ml/min, use Qd=800ml/min

To prescribe adequate dialysis, please consult the following chart for a guide. Hours required to achieve Kt/V>1.3

Always round up to the higher time and assume that Qb initially at least will be less than you prescribe

Work on the principle that the average dialysis duration is ~4.5 hrs – start with this and then tailor to the patient on the basis of access performance & measured dialysis adequacy

Do not prescribe less than 3 hours three times a week except in exceptional circumstances (discuss with named consultant)

Maximise Qb – particularly important in larger patients

Do not use FX8 in large people or FX10 in small people (though FX60 may be appropriate – discuss with named consultant)

High flux dialysis (with FX60 dialyser) or haemodiafiltration (with FX80 and adaptation to dialysis machine) may be appropriate for anuric patients on (or expected to be on) dialysis for a prolonged period or with signs/symptoms of dialysis amyloid to aid middle molecule clearance

HDF may also be appropriate for larger patients with very long hours (discuss with named consultant)

This page last modified on Oct 29, 2018 @ 10:30 am.
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